Recovery of silver-containing scales from aqueous media

ABSTRACT

A method is disclosed for controlling the deposition of metal-containing scales, such as iron silicate scale, from a hot, aqueous, geothermal brine or the like, without substantial corrosion of brine handling equipment. The brine is contacted with (1) an amount of an acid sufficient to reduce the pH of the brine between 0.1 and 0.5 unit and (2) a greater than stoichiometric amount of a reducing agent for reducing trivalent iron and manganese cations in a high temperature brine solution to divalent ions. An overall decrease in scale deposition, especially of iron silicate scale, is achieved while a silver-rich scale can be recovered from silver-containing brines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the treatment of a hot aqueous brine solutioncontaining various dissolved components, such as iron, silver andsilica, to inhibit precipitation of undesirable scale, such as ironsilicate scale, therefrom while enhancing deposition and recovery of avaluable silver-containing scale. More particularly, the inventionrelates to such a treatment wherein the scale is formed when the brineis produced and handled in a manner so that its temperature and pressureare reduced, as when a geothermal brine is processed to recover its heatcontent.

2. Description of the Prior Art

The solubility of most ions in solution decreases with a decrease intemperature or pressure of the solution. If dissolved ions are presentnear their saturation concentration in the solution, a slight reductionin the temperature or pressure of the system can result in precipitationof a portion of these ions, which often combine and deposit as a scaleon any solid surface with which they come into contact, such as thevessel or conduit in which the solution is confined.

One example of such a solution is a liquid stream containing hot waterwhich is passed through a conduit in an industrial operation underconditions, such as a lowering of the pressure, which flash at least aportion of the hot water to steam. If the hot water is a brinecontaining appreciable amounts of dissolved salts, this flashing isoften accompanied by the formation of scale on the surfaces of theconduit contacted by the fluid stream. Scale deposits tend to build upover a period of time and restrict further fluid flow through theconduit requiring either operation at a reduced flow rate or an increasein the amount of power used to move the fluid through the conduit. Inextreme cases, the conduit can become completely plugged with scale andthe industrial operation must be shut down for maintenance. Industrialoperations for generating steam power often are hampered by the buildupof scale deposits caused by flashing of hot water containing dissolvedsalts. Among the various methods used to produce power from steam arefossil-fuel steam generators, nuclear steam supply systems, andgeothermal generator units.

Geothermal steam and hot brines are found in naturally occurring, largesubterranean reservoirs in many regions of the world. If located atreadily accessible sites, geothermal steam and water or brine have, forsome time, been used for therapeutic purpose, for industrial processes,or for direct heating. Although interest in developing geothermalresources further for these purposes still exists, recently theprincipal effort has been towards developing these partially renewableresources for production of electric power.

Techniques are known whereby hot geothermal fluids can be used togenerate electric power. Pressurized geothermal water or brine, having atemperature above about 400° F., can be flashed to a lower pressure andthe steam generated by flashing can be used to drive a steam turbine incombination with an electric generator. However, formidable problems aregenerally encountered in handling and disposing of large amounts ofheavily contaminated and frequently highly saline geothermal liquids.Consequently, production of electricity from geothermal waters on acommercial scale has been difficult and costly to achieve.

One of the most serious problems encountered in using hot geothermalliquids for producing electric power results from scaling of theequipment used to confine and contain the liquid. Because geothermalliquids have usually been confined in subterranean reservoirs forextraordinarily long periods of time at elevated temperatures, largeamounts of minerals are leached from the reservoirs into the brine.Typically, salts and oxides of heavy metals, such as lead, zinc, iron,silver, cadmium and molybdenum, are found in geothermal brine. Othermore common minerals, such as calcium and sodium, are also dissolved inthe brine, as are naturally occurring gases, including carbon dioxide,hydrogen sulfide and methane. An especially troublesome component of thehot brine may be silica, which is found in large concentrations in theform of silicic acid oligomers.

Various proposals have been made to decrease the scale formation inequipment used in producing and handling geothermal brine. In "FieldEvaluation of Scale Control Methods: Acidification," by J. Z. Grens etal, Lawrence Livermore Laboratory, Geothermal Resources Council,Transactions, Vol. 1, May 1977, there is described an investigation ofthe scaling of turbine components wherein a geothermal brine at apressure of 220 to 320 p.s.i.g. and a temperature of 200° to 230° C.(392° to 446° F.) was expanded through nozzles and impinged againststatic wearblades to a pressure of 1 atmosphere and a temperature of102° C. (215° F.). In the nozzles, the primary scale was heavy metalsulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide andcuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphoussilica promote adherence of the primary scale to the metal substrate. Bycontrast, the scale formed on the wearblades was cuprous sulfide, nativesilver and lead sulfide in an iron-rich amorphous silica matrix. Whenthe brine which originally had a pH of 5.4 to 5.8 was acidified withsufficient hydrochloric to reduce the pH of the expanded brine to valuesbetween 1.5 to 5.0, scaling was eliminated. However, acidification ofhot brines promotes corrosion of the brine-handling equipment to suchlevels that corrosion defeats the use of acid for scale control.

It is known to recover metal values and salts from brine, such asgeothermal brine produced from a subterranean reservoir. U.S. Pat. No.4,127,989 to Mickelson discloses a method in which brine is pressurizedand maintained above the bubble point pressure and thereafter aprecipitating agent such as a soluble sulfide, is added to the brine toenhance formation of insoluble metal sulfide precipitates. Soluble saltsand metal values are recovered from the brine effluent after the hotbrine has been processed to recover energy therefrom. Silver sulfidesare among the mineral values recovered by this process.

While the aforementioned treatments have met with some success inparticular applications, the need exists for a further improved treatingprocess to reduce scale deposition during the handling of hot aqueousbrines, especially geothermal brines.

Accordingly, it is a principal object of this invention to provide amethod for inhibiting the buildup of scale on surfaces of the fluidhandling equipment contacted by a hot water-containing fluid stream andfor removal of such scale without accelerating corrosion of the fluidhandling equipment.

It is a further object of this invention to provide a method forinhibiting the deposition upon fluid handling equipment of undesirablemetal-containing scales, especially metal-silicate scale, from ageothermal brine while minimizing corrosion of the handling equipment.

It is another object of this invention to treat a geothermal fluid,containing at least a portion of a geothermal brine, utilized for thegeneration of electric power so as to inhibit the deposition of metalsilicate scale from the geothermal brine onto the fluid handlingequipment while minimizing corrosion the fluid handling equipment.

Yet another object of this invention is to promote deposition andrecovery at certain locations within the fluid handling equipment ofvaluable silver-containing scales, especially of silver antimonidescales from geothermal brines containing substantial concentrations ofsilver, while inhibiting the overall precipitation of scale upon thefluid handling equipment without substantial corrosion thereof.

Other objects, advantages and features of the invention will be apparentfrom the following description, drawing and appended claims.

SUMMARY OF THE INVENTION

The present invention provides a method for inhibiting the deposition ofscale from an aqueous fluid, such as a geothermal brine, comprisingadding to the fluid an acid or acid precursor in an amount sufficient toreduce the pH of the fluid by between about 0.1 and 0.5 pH unit andfurther adding to said fluid a reducing agent, in particular, sodiumformate.

In one embodiment of the invention, there is provided a method fortreating a stream of pressurized hot water or brine containing trivalentmetal ions, especially those of iron and manganese, together with silicaspecies, which stream is passed through one or more vessels or conduitsin which the pressure is reduced so that at least a portion of the waterin the stream is flashed to produce steam. An acid soluble in water inan amount sufficient to lower the pH of the aqueous solution between 0.1and 0.5 unit is added to the liquid stream in combination with a greaterthan stoichiometric amount of a reducing agent for reducing thetrivalent iron and manganese cations contained in the brine to divalentcations so that corrosion is minimized and the overall formation ofscale in the vessel or conduits, especially iron silicate scale, isinhibited, and any formed scale is washed away.

In yet another embodiment of the invention, a method is provided whereinan aqueous fluid, such as a geothermal brine, which contains silver,usually in dissolved form, and which further contains scale-formingconstituents, is treated so as to produce a silver-containing scale. Inthis embodiment, an acid or acid precursor is added to the fluid incombination with a reducing agent, such as sodium formate, and when thefluid is then subjected to scale-forming conditions, such as asubstantial reduction in pressure in a relatively short amount of time,e.g., steam flashing conditions, a scale forms which is relatively richin silver. In this embodiment of the invention, particularly strikingresults have been obtained when the geothermal brine or other fluidcontains antimony in addition to silver. And it has been further foundthat the silver-rich scale can be induced to form in relatively largequantities in locations where there is high turbulence.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be more readily understood by reference tothe drawing which depicts in simplified form relevant portions of anexemplary geothermal brine production well and power plant with whichthe method for scale control of the present invention may be used toadvantage.

DETAILED DESCRIPTION OF THE INVENTION

In producing and utilizing hot pressurized brine solutions which containdissolved metal salts at or near their saturation concentration so thatthe pressure and/or temperature of the solutions are reduced, a portionof the metal salts can precipitate and deposit as scale on the surfacesof the vessel or conduit confining the brine. Examples of such hotpressurized brine solutions include geothermal brines and other brinesused in a wide variety of industrial operations. A number of differenttypes of scale can form depending on the nature and concentration of themetal salts in solution. Many of these scales are soluble in an acidsolution. However, the addition of acid into the system usually resultsin severe corrosion of the ferrous metals normally used in themanufacture of the confining vessels and conduits because the brine isusually at high temperature, for example, between 250° and 500° F. Whilecorrosion inhibitors are known for most acids, these inhibitors do notfunction well under the severe temperature conditions of many systems.Thus, the use of acid to dissolve such scale has been limited and mostoften is confined to systems employing corrosion resistant but expensivemetals rather than the ferrous metals found in most industrialequipment.

The concentration of ferric ions and silica species contributed by ironand silica containing minerals dissolved from the reservoir by the hotbrine is normally high so that iron silicates are among the mostpervasive and troublesome scales formed in the equipment used to handleand process geothermal brines. Corrosion of ferrous metals by additionof acid to the system further increases the concentration of ferric ionspotentially available to form precipitates of iron silicates.

It has now been found that scale formation from brine can be reduced andscale previously formed can be removed by including in the brine an acidin an amount necessary to lower the pH of the brine between 0.1 and 0.5unit. To minimize the increased corrosion of ferrous metal componentsthat occurs upon the addition of even these small amounts of acid, areducing agent is added, usually in an amount greater than thatstoichiometrically required to reduce the trivalent iron and manganesecations contained in the fluid stream to divalent cations. The reducingagent not only minimizes corrosion, but also reduces the formation ofscale by reducing the number of trivalent iron and manganese cationscontributed to the brine during corrosion from the containing vesselsand conduits.

Shown in simplified form in the drawing are relevant portions of anexemplary geothermal brine power plant 10. Comprising generally powerplant 10 are first wellhead separator 12, second wellhead separator 14and flash vessel 16. Shown included in power plant 10 are gas cleaningmeans 18 and steam turbine/generator 20. Associated with power plant 10,but not actually forming a part thereof, are brine extraction well 28and brine reinjection well 26. Extraction well 28 penetrates into earth24 a depth "d₁ " to brine producing formation 30 which has a thicknessof "d₂." Ordinarily, reinjection well 26 is similar to extraction well28, and may penetrate to the same producing formation 30.

In operation, hot geothermal brine is introduced under pressure fromextraction well 28 through conduit 32 into the side region of firstseparator 12. Within separator 12, non-condensible gases, includinghydrogen sulfide, carbon dioxide and ammonia, are stripped from thegeothermal brine. These non-condensible gases are discharged from thetop of separator 12 through gas conduit 38 into gas cleaning means 18.The brine is discharged from the bottom of separator 12 and is directedthrough conduit 40 into the side region of second separator 14, in whichremaining amounts of non-condensible gases are stripped from the brine.These remaining gases are discharged from the top of separator 14through gas conduit 42 into conduit 38, to be directed thereby into gascleaning means 18. Included in gas cleaning means 18 may be heatexchangers (not shown) which enable heat to be extracted from the hotnon-condensible gases to create additional steam from steam condensatein a manner known to those skilled in the art.

From separator 14, the brine is discharged through conduit 44 intoflashing vessel 16. Within flashing vessel 16, the brine is flashed to asubstantially lower pressure, for example, from an initial pressure ofabout 450 p.s.i.g. to the lower pressure of about 150 p.s.i.g., so as torelease steam, the released steam being routed from vessel 16 throughconduit 46 to steam turbine/generator 20. Condensate and/or steam aredischarged from turbine/generator 20 through conduit 48 for disposal orfor routing to heat exchanger portions of gas cleaning means 18. Flashedbrine is discharged from flashing vessel 16 through conduit 54 to pump56 which pumps the brine through conduit 58 into reinjection well 26.Alternatively, pump 56 may pump the brine to other means of disposal orto other uses (not shown).

Effective pH of the brine as it enters well 28 from formation 30 isbelieved typically to be between about 4 and 4.5; however, due toremoval of the non-condensible gases, the pH of the brine typicallyincreases to between about 5 and 6 by the time it enters flashing vessel16. Ordinarily, as the brine enters well 28 from formation 30, flashingoccurs to an extent causing release of about 1 to 1.5 percent of thesteam contained in the brine, and by the time the brine reaches the topof well 28, additional flashing usually has occurred to an extent thatbetween about 10 and 20 percent of the steam has been released. Brinetemperature at producing formation 30 varies considerably from well towell, but is usually in the broad range of from about 350° to 600° F.,with a brine temperature of between about 450° to 500° F. being typicalof many localities.

Any convenient means of introducing the acid and reducing agent to thebrine can be used. However, when an acid and reducing agent are selectedwhich evolve gases (e.g., hydrochloric acid and zinc metal, hydrochloricacid and potassium cyanide), care should be taken to avoid evolution ofgases during the mixing of the acid and reducing agent, for example, bymixing the reducing agent upstream of the acid component or by mixingthe reducing agent into a relatively dilute acidic solution. Usually,however, a mixture of acid and reducing agent is introduced from source60, through conduit 62 containing valve 64 into conduit 66 which extendsdown well 28, inside of well casing 68, terminating in nozzle 70positioned approximately opposite brine producing formation 30. In apreferred embodiment, the downhole apparatus employed in the method ofthis invention includes anchor 72 attached to nozzle 70 by connectingrod or conduit 74. anchor 72 helps maintain the position of nozzle 70 inwell 28 during the injection of the mixture of acid and reducing agentdown conduit 66. Conduit 66 can be a small diameter coiled tubingextending several thousand feet down well 28 from wellhead 76, which inthe absence of anchor 72, may permit nozzle 70 to move about undesirablyin well 28. Anchor 72 helps maintain nozzle 70 adjacent to brineproducing formation 30, the location where it is desired to inject themixture of acid and reducing agent into the flow of geothermal brine.For convenience, anchor 72 may be positioned anywhere in the vicinity ofnozzle 70. Since brine producing formation 30 is sometimes incompetent,it is preferred to position anchor 72 in well 28 below producingformation 30 as shown in the drawing. Anchor 72 can be of anyconventional design, for example, an anchor having three or more lockingarms 78 which fold up independently as collars while anchor 72 is beinglowered downhole. To set locking arms 78 against the formation when thedesired depth is reached, conduit 66 is retracted a short distance sothat the locking arms unfold. Later, when it is desired to remove nozzle70 from the well, a stronger upward pull on conduit 66 shears a pin (notshown) in anchor 72, allowing locking arms 78 to collapse and theapparatus to be pulled from well 28. Since the acid exits conduit 66 vianozzle 70, connecting rod or conduit 74 does not transport any fluid.The purpose of conduit 74 is merely to attach nozzle 70 firmly to anchor72. Conduit is used in this embodiment to attach the anchor to thenozzle simply because conduit material is readily available and providesmechanical strength.

Since the portion of conduit 66 extending below wellhead 74 is exposedto the high temperature environment of the geothermal fluid beingproduced, it is preferred that conduit 66 and nozzle 70 be made of acorrosion resistant metal, e.g., stainless steel, Hastelloy, Inconel, orthe like.

While a mixture of acid and reducing agent can be injected from source60 downhole at producing formation 30 so as to lower the pH of the brineas close as is practical to its origin, the pH of the brine usuallyincreases further as it passes through power plant 10, for example, asnon-condensible gases are removed at separators 12 and 14, and as thebrine is flashed in vessel 16. Thus, it is often preferred to inject themixture of acid and reducing agent into the brine flow in places wheresignificant pH increases otherwise occur. By so doing, a more uniformbrine treatment may be achieved. Alternatively, the amount of brinetreatment may be varied according to the amount required at variouslocations.

To this end, in addition to or rather than being injected downhole, acidor reducing agent or a mixture of both may be injected at any of thefollowing locations: (1) into brine conduit 32 between wellhead 76 andfirst separator 12 via conduit 80 containing valve 82; (2) into brineconduit 40 between first and second separators 12 and 14 via conduit 84containing valve 86; (3) into brine conduit 44 between second separator14 and flash vessel 16 via conduit 88 containing valve 90; and/or (4)into conduit 58 just upstream of injection well 26 via conduit 92containing valve 94. Conduits 80, 84, 88 and 92 each are connected toacid source 60 (connections not shown).

While the treatment using acid and reducing agent of this invention iseffective in controlling a wide variety of scale, of particularimportance are metal silicate scales, especially iron silicate scales.Such scales are believed formed by reaction of hydrated ferricoxyhydroxide with silicic acid or silicic acid oligomers as follows:##STR1## Further acidizing the already acidic geothermal brine isbelieved to shift equilibrium conditions away from the formation of aprecipitate and/or to interfere with the precipitation reactioninvolved. The hydrogen ions (H⁺) added to the brine by addition offurther acid are believed to tie up the ferric oxyhydroxide and therebyinhibit the indicated reaction of ferric oxyhydroxide with silicic acidwhich forms insoluble iron-rich silicates. The addition of reducingagents capable of reducing trivalent iron and manganese cations isbelieved to further inhibit the formation of the precipitate byinterfering with the formation of ferric oxyhydroxide and other metaloxyhydroxides. However, regardless of the specific nature of thereaction involved and the specific effects upon the reaction of the acidand reducing agent, it has nevertheless been demonstrated that themethod of this invention is effective in reducing the scales depositedfrom geothermal brine while inhibiting the corrosion of the metalvessels and conduits.

The brine-soluble acids suitable for use in this invention are inorganicmineral acids, organic carboxylic acids, mixtures thereof, andcombinations of inorganic and organic acids. Mineral acids which may beused are hydrochloric, sulfuric, nitric and perchloric acid. Suitableorganic carboxylic acids are those that form water-soluble oracid-soluble salts of alkali metals and alkaline earth metals. Aromaticand aliphatic monocarboxylic, dicarboxylic and tricarboxylic acidshaving about 1 to 6 carbon atoms can also be used. The carboxylic acidscan be saturated or unsaturated and substituted or unsubstituted. When asubstituted carboxylic acid is used, the most common substituent is thechloride ion. For example, benzoic, formic, acetic, chloroacetic,peracetic, trichloroacetic, citric, oxalic and maleic acids can be used.The most preferred brine-soluble acid is hydrochloric acid. The acid isgenerally added in an amount sufficient to lower the pH of the brine bybetween 0.1 and 0.5 pH unit, and preferably by between 0.3 and 0.4 pHunit. Generally, about 50 to 180 weight parts per million of awater-soluble acid is employed. Higher concentrations of acid caninterfere with the activity of certain of the reducing agents listedhereinafter. While any of the acids may be used in concentrated form, itis common to employ aqueous solutions of such acids. For example, anaqueous solution containing about 31 percent by weight hydrochloric acidis often employed.

The brine-soluble reducing agents suitable for use in this invention arethose capable of reducing scale-forming trivalent transition metal ionsto divalent ions, especially ions of the metals which form silicatescales, such as iron and manganese. Exemplary organic reducing agentsinclude carbon monoxide, sodium formate, formaldehyde, dextrose,sucrose, corn syrup, glyoxal, acetaldehyde, butyraldehyde, methanol,ethylene glycol, t-butanol, phenol, hydroquinone, potassium cyanide,carbon disulfide, thioglycolic acid, ammonium thioglycolate urea, ureahydrochloride, formamide, formamide hydrochloride, ammoniumthiocarbonate, thiourea, ascorbic acid, formic acid and oxalic acid.Among the inorganic compounds useful as reducing agents in thisinvention are potassium iodide, sulfur, sodium thiosulfate, sodiumdithionite, stannous chloride, iron wire, aluminum, hydrazinehydrochloride, sulfur dichloride, arsenious acid, hydrogen, sodiumsulfite, sodium bisulfite, sulfur monochloride, sodium and ammoniumthiosulfates, elemental iron dust, and elemental zinc dust. Thepreferred reducing agents are sodium formate, thiosulfates, elementaliron dust, elemental zinc duct, and sodium dithionite with the mostpreferred reducing agent being sodium formate.

The concentrations of dissolved metals in geothermal brines vary fromlocation to location so that the metal cations capable of precipitatingas silicates at any given geothermal site may include silicate-formingcations in addition to the trivalent cations of iron and manganese, suchas perhaps those of vanadium and cobalt. However, in the vast majorityof geothermal brines, the concentrations of silicate-forming cations inaddition to those of iron and manganese are extremely small. For thisreason it has been found that generally the reducing agent should beadded in an amount at least slightly greater than thatstoichiometrically required to reduce to divalent cations the trivalentiron and manganese cations contained in the brine, the excess beingsufficient to also reduce any trace amounts of other silicate-formingcations that may be present in the brine. The concentrations oftrivalent iron and manganese cations in geothermal brine may easily bedetermined by known means. Typically the amount of reducing agent usedis between one and two times, and preferably is two times, thestoichiometric amount required to reduce the trivalent iron andmanganese cations in the fluid stream to divalent cations. For example,if the geothermal fluid contains (as many brines from Brawley, Calif.do) 5 to 50 wppm of trivalent iron cations and less than 5 wppm oftrivalent manganese cations, the stoichiometric concentration of sodiumformate needed is 3 to 33.5 wppm, so that, in the preferred embodiment,sufficient sodium formate is added to the fluid to provide aconcentration of twice this value, i.e., 6 to 67 wppm.

The present invention is further described by the following examples,which are illustrative of various aspects of the invention but which arenot intended as limiting the scope of the invention as defined by theappended claims.

EXAMPLES 1 to 7

A series of field tests are made to determine the effect of the additionof hydrochloric acid to a high enthalpy, silica-rich, heavy metal laden,hypersaline brine of an extremely reactive nature. There is producedfrom a production well a geothermal fluid comprising a mixture of steamand brine. The geothermal fluid is passed through a first separatorwhere the steam and brine are separated. The steam, which would normallybe used to drive a turbine, is vented to the atmosphere at this testfacility. Various amounts of acid are then injected into the flowingstream of brine just downstream of the first separator. The acidifiedbrine is next passed through a second separator where the pressure isreduced to flash additional quantities of steam, which is also vented tothe atmosphere. The acidified brine is then repressurized and reinjectedback into the subterranean geothermal reservoir via an offset injectionwell. During the test, measurements are made of the pH of the brinebefore and after the addition of the acid, the extent of scale buildupin the unit, and the extent of corrosion at various locations in thefluid handling equipment.

These field tests and the results therefrom will now be described inmore detail. A geothermal fluid comprised of about 90 percent by weightbrine and about 10 percent by weight steam is produced from a productionwell at a temperature of 455° F. and a pressure of 400 p.s.i.g. Thebrine contains about 251,000 weight parts per million of variouselements: less than 0.3 aluminum, 1.7 silver, 12.8 arsenic, 319 boron,1,070 barium, 103 bromine, 1.2 cadmium, 25,000 calcium, 149,000chlorine, 0.06 chromium, 12 cesium, 5.2 copper, 0.4 fluorine, 459 iron,5 iodine, 13,500 potassium, 1,770 lithium, 49 magnesium, 793 manganese,58,000 sodium, less than 0.1 nickel, 81 lead, 73 rubidium, less than 1selenium, 200 silicon, 400 strontium, and 302 zinc.

The geothermal fluid is passed at a rate of about 250,000 pounds perhour through an 8 inch diameter carbon steel pipe to and through two 48inch diameter, 26 feet long horizontally positioned separators operatedin series. The first separator is operated at 400 p.s.i.g. The separatedsteam, which would normally be used to drive a turbine, is vented to theatmosphere at this test facility. Just downstream of the first separatoris positioned a 10 inch diameter, 41 inch long acid mixing spoolcontaining a three element vane-type motionless mixer. A 31 percent byweight aqueous solution of hydrochloric acid is pumped at various ratesfrom a storage facility into the acid mixing spool via an injectionnozzle which is a Hastelloy pipe extending into the acid mixing spoolnear the brine inlet end thereof.

The acidified brine is next fed to the second horizontally positionedseparator which is operated at 200 p.s.i.g. The additional separatedsteam is again vented to the atmosphere. The acidified brine is next fedto a charging pump where the pressure of the brine is increased to about250 p.s.i.g. The partially-repressurized acidified brine is then fed toan injection pump which pumps the brine at a high flow rate and at about750 p.s.i.g. pressure approximately 5,000 feet to an offset injectionwell and back into the geothermal reservoir. Carbon steel corrosioncoupons are positioned in the flowing brine at four locations, i.e.,just before the acid injection point, just after the acid injectionpoint, immediately downstream of the injection pump discharge and nearthe injection wellhead.

The pH of the brine is measured above and below the acid mixing spool.

Scale buildup on piping and vessels is measured by online radiographictechniques employing an Iridium-192 source. The source and film plateare positioned on opposite sides of the of interest. The differences ingamma absorption of the brine, scale and steel results in sufficientcontrast to produce a radiograph from which projected thickness of scalecan be measured.

The results of these tests are summarized in Table I. In the absence ofthe addition of acid and reducing agent, scale builds up rapidly in thetest apparatus. Upon addition of enough hydrochloric acid to decreasethe pH of the flowing brine stream up to about 0.3 pH unit, the buildupof scale is substantially decreased without substantial increase incorrosion of the equipment handling the brine. Increasing the amount ofhydrochloric chloric acid sufficiently to lower the pH of the flowingbrine stream from 0.43 to 0.80 pH unit further substantially increasescorrosion in the system. Thus, an amount of hydrochloric acid sufficientto lower the pH of the brine stream up to about 0.3 pH unit withoutaddition of reducing agent causes only moderate corrosion of theequipment handling the brine. However, at some point as the decrease inpH increases from 0.3 to 0.43 unit, the corrosion becomes severe if acidalone is used.

                                      TABLE I                                     __________________________________________________________________________    EFFECT OF ADDITION OF HYDROCHLORIC ACID TO HOT FLOWING GEOTHERMAL BRINE                  Amount Hydrochloric                                                           Acid Added                                                         Example                                                                            Length of                                                                           (weight parts                                                                            pH Upstream                                                                           pH Downstream                                                                          pH Reduction                                                                         Scale                           Number                                                                             Test (days)                                                                         per million)                                                                             of Mixing Spool                                                                       of Mixing Spool                                                                        Across Spool                                                                         Formed                                                                             Corrosion                  __________________________________________________________________________    1    8     none       5.97    5.97     0.00   heavy                                                                              moderate                   2    5     147        5.97    5.80     0.17   moderate                                                                           moderate                   3    8     189        5.95    5.67     0.28   moderate                                                                           moderate                   4    3     214        5.98    5.68     0.30   moderate                                                                           moderate                   5    13    253        5.88    5.45     0.43   moderate                                                                           severe                     6    5     358        5.92    5.24     0.68   moderate                                                                           severe                     7    2     400        6.30    5.47     0.83   light to                                                                           severe                                                                   moderate                        __________________________________________________________________________

EXAMPLES 8 to 13

To compare the effect upon the deposition of scales from high enthalpy,heavy metal laden, geothermal brine caused by adding acid alone withthat caused by adding acid in combination with a reducing agent, aseries of six pilot scale tests are conducted.

The apparatus utilized in the tests comprises a three-inch conduit intowhich is introduced a portal for injecting the additives and along whichare positioned corrosion/scaling spools and a static mixer to providethe turbulence needed to mix the additives into the brine. The staticmixer is a 12 inch section of 2-inch conduit housing a helix formed ofstainless steel through which the fluid is forced as it flows along thethree-inch conduit. The corrosion/scaling spools are short sections of2-inch conduit which hold conventional corrosion strips to monitor thecorrosivity of fluids downstream of the additive injection point andalundum ceramic balls to provide turbulence and surface area upon whichscale will preferentially deposit. A pressure reducing valve and flashseparator vessel are also positioned along the three-inch conduit, tocause flashing of the brine, each being located upstream of a set ofcorrosion/scaling spools.

A geothermal brine comprised of about 90 percent by weight brine andabout 10 percent by weight steam flows by gravity into the three-inchconduit through an inlet conduit of two-inch insulated pipe.Concentrations of components in the brines entering the unit fall withinthe following ranges of concentrations established for the brines fromBrawley, Calif. These ranges were established by compiling values fortwo representative Brawley brines, one containing the least and onecontaining the most dissolved salts found in the area: aluminum lessthan 2; antimony less than 2; arsenic 2.5, barium 680-1,520, boron170-340, calcium 12,700-34,000, chloride 80,200-184,000, copper 1.7-4,iron 490-4,800, lead 17-300, magnesium 100-200, manganese 470-1,860,potassium 7,300-17,100, rubidium 34-83, silicon 270-320, silver lessthan 0.6, sodium 35,000-68,000, strontium 640-1,510, tin less than 10,zinc 190-1,170 and total dissolved solids 128,000-262,000. (All of theforegoing values are reported in milligrams per liter.)

In each of the six tests, hydrochloric acid alone followed byhydrochloric acid in combination with sodium formate is injected throughthe additive portal using a portable high pressure injection pumpcapable of generating a pressure differential of 800 p.s.i.g. To assuresteady flow of additives, the injection pump is started before brine ispermitted into the unit. The brine solution has an average temperatureof 425° F. and a pressure of 450 p.s.i.g. just downstream of theadditive portal. The flow rate of the process fluid as measured by atwo-inch metering orifice located down-stream of the static mixer is 750pounds per hour. Downstream of the metering orifice, a 3/4-inch flowcontrol valve provides a pressure drop of approximately 400 p.s.i.g.,which is sufficient to cause flashing of the fluid stream. Justdownstream of the flow control valve is located a stream jacketedsection of conduit having a residence time for fluid passingtherethrough of 15 seconds, which simulates the 10-inch header in thewell facility. Corrosion/scale spools locted within the steam-jackectedsection collect the scale formed as a result of the flashing andturbulence caused by the pressure-reducing flow control valve.

Upon exiting the steam-jacketed conduit containing corrosion/scalespools, the two phase brine enters a flash separator vessel, in which itflashes a second time and from which steam is vented overhead. Secondaryflashing of the brine is caused by a pressure drop across the flashvessel of 300 p.s.i.g. The supersaturated liquid brine flows from thebottom of the flash vessel and as described above is injected via asecond injection portal with additional acid or acid and reducing agentas needed to maintain the pH of the solution at the desired level. Afterpassing the second injection portal, the brine passes into a section ofthree-inch conduit containing more corrosion/scale spools of a lengthsufficient to provide a residence time of 14 minutes for the brineflowing therethrough. This section of conduit simulates the lengthyinjection conduit in the field, which returns processed brine from thesteam plant back to the injection well. At the exit from the simulatedinjection conduit the brine is at a temperature of 370° F. and apressure of 135 to 140 p.s.i.g.

In separate tests, hydrochloric acid alone and hydrochloric acid incombination with sodium formate are added to geothermal brine and thescales deposited upon flashing the brine are analyzed to determine ineach instance the amount of total scale formed and the iron silicatecontent of the scale. The results are summarized as Examples 8 to 13 inTable II. In Examples 8 through 13, for the tests employing acid alone,sufficient acid is added to drop the pH of the brine by 0.5 unit. Inthese same examples, for the tests employing acid in combination with areducing agent, sufficient acid is added to drop the pH of the brine by0.5 unit and two times the stoichiometric amount of sodium formatenecessary to reduce the trivalent iron and manganese cations in thefluid stream to their corresponding divalent states is added as thereducing agent.

The results summarized in Table II compare the amount of total scale andthe amount of iron silicate scale formed in

Examples 8 to 13 using acid alone versus acid plus reducing agent. Theunits in Table II are parts of constituent formed (i.e., total scale,iron silicate, etc.) per million parts by weight of brine flowingthrough the test facility. These figures are obtained by dismantling thepilot test facility, scraping and collecting the scale from its wallsand surfaces, especially from the corrosion/scale spools, and submittingthe scale to chemical analysis to determine its content.

                                      TABLE II                                    __________________________________________________________________________    COMPARISON OF SCALE PRODUCED UPON ADDITION OF                                 HYDROCHLORIC ACID VS. HYDROCHLORIC ACID PLUS REDUCING                         AGENT TO HOT FLOWING GEOTHERMAL BRINE                                         Acid Alone            Acid and Reducing Agents                                (weight parts per million)*                                                                         (weight parts per million)                                 Total                                                                              Iron                                                                              Silver                                                                              Total                                                                             Total                                                                              Iron                                                                              Silver                                                                              Total                                    Test                                                                             Deposit                                                                            Silicate                                                                          Antimonide                                                                          Silver                                                                            Deposit                                                                            Silicate                                                                          Antimonide                                                                          Silver                                   __________________________________________________________________________     8 0.7  0.6           0.8  0.6                                                 9 0.7  0.6           0.4  0.3                                                10 27.0 23.0          19.0 12.0                                               11 9.1  7.4           11.0 8.0                                                12 4.5  2.6 1.1   0.5 6.3  2.7 2.3   1.2                                      13 3.3  2.3 0.5   0.2 3.3  1.6 1.3   0.7                                      __________________________________________________________________________     *weight parts per million weight parts of brine                          

These results show that addition of a combination of acid and a reducingagent in tests 9, 10 and 13 decreases production of iron silicate scaleby up to 50 percent over that produced with addition of acid alone.Additionally, in tests 12 and 13, the results of which are summarized inTable III, it was determined by metallurgical examination of thecorrosion strips held within the corrosion/scale spools that acid usedin combination with a reducing agent substantially decreases corrosionover that which results when acid is used alone. The total amount ofmetal lost in general corrosion and the rate at which pits grow are bothdecreased, but especially the latter, which occurs only one fourth asfast with addition of reducing agent plus acid as with addition of acidalone. Thus, the present invention is useful for reducing both scalingand corrosion, the latter by at least about 10 percent for generalcorrosion, and by at least about 50 percent, and often by at least about75 percent, for pitting corrosion as compared to the use of acid alone.

                  TABLE III                                                       ______________________________________                                        COMPARISON OF CORROSION PRODUCED UPON                                         ADDITION OF HYDROCHLORIC ACID VS. HYDRO-                                      CHLORIC ACID PLUS REDUCING AGENT TO HOT                                       FLOWING GEOTHERMAL BRINE                                                      Acid Alone       Acid and Reducing Agent                                      (ml/yr)          (ml/yr)                                                           General             General                                              Test Corrosion   Pitting Corrosion  Pitting                                   ______________________________________                                        12   258         2385    224         554                                      13   306         4524    254        1099                                      ______________________________________                                    

The reducing agent serves the dual purpose of enhancing theeffectiveness of the acid as a scale inhibitor and of reducing thecorrosive effects of the acid on the ferrous components in the confiningmetal vessels and containers. The results summarized in Table II showthat the overall amount of scale formed with the addition of acid andreducing agent is comparable to or less than that found when acid aloneis added, and in the case of iron silicate scale the amount formed uponaddition of acid and reducing agent is comparable to or up to 50 percentless than that formed upon addition of acid alone.

A review of the data in Tables I, II, and III reveals that in theabsence of both acid and reducing agent there is a significant increasein the formation of scale from geothermal brines, while addition of acidin combination with a reducing agent not only achieves a reduction inscale formation comparable to or better than that achieved by additionof acid alone, but also significantly inhibits the corrosive effect ofthe acid upon the brine handling equipment, even when the pH of thebrine is reduced by as much as 0.3 to 0.5 unit. The method of thisinvention, therefore, is highly effective for reducing scale formed fromgeothermal brines without severely corroding the brine handlingequipment necessary to recover energy from the brine.

Particularly to be noted in Examples 12 and 13 in Table II is thegreatly increased amount of silver-containing scale produced withaddition of a combination of acid and reducing agent as compared withthe amount produced when acid is used alone. In these experiments, theamount of total silver and of silver antimonide formed in the presenceof hydrochloric acid and a greater than stoichiometric amount ofreducing agent is increased by greater than 100 percent over thatproduced with addition of acid alone. The concentration of silver in thesilver-containing scale formed is also 1.5 times as great and, in thecase of test 13, is 3.5 times as great. Almost all of thesilver-containing scale reported in Examples 11 and 12 of Table II formsas silver antimonide.

Generally, the concentration of silver in geothermal brines suitable foruse in this invention will be between about 0.1 and 10.0 wppm, andusually above about 0.3 wppm, with a silver concentration above about 2wppm being relatively rare. The concentration of silver in geothermalbrines from Brawley, Calif. is usually at or above about 0.5 wppm.

The location at which silver-containing scale will deposit also dependsupon the type of scale-reducing treatment used. In general, scale ingeothermal systems forms when the brine is subjected to flashing orother scale-forming conditions wherein a supersaturated solution ofdissolved solids forms in a liquid phase, followed by precipitation ofscale. In the usual case, the constituents of the scale deposit randomlyin the brine handling equipment. For example, with the addition of acidalone, it has been found that silver deposits in a random,non-preferential manner. However, the scale formed upon addition of acidin combination with reducing agent deposits silver preferentially atlocations within the brine stream of high turbulence, such asimmediately downstream of pumps and valves.

Although the invention is not to be held to any particular theory ofoperation, it is believed that, in the presence of a reducing agent anda brine containing dissolved antimony, the silver chloride formed uponaddition of hydrochloric acid undergoes the following reactions.

    e.sup.- +AgCl→Ag+Cl.sup.-  (with reducing agent)    (1)

    3e.sup.- +2H.sup.+ +SbO.sup.+ →Sb+H.sub.2 O (with reducing agent) (2)

    xAg+ySb→Ag.sub.x Sb.sub.y                           (3)

This set of reactions is favorable at locations of high turbulencewithin the fluid handling equipment. The turbulence, it is believed,imparts sufficient kinetic energy to make Reaction (3) thermodynamicallyfavorable. However, regardless of the specific nature of the reactionsinvolved and the specific effects of the reducing agent or theturbulence, it has nevertheless been demonstrated that using the scaleand corrosion reducing method of this invention greatly increases boththe amount of silver-containing scale produced and the total silvercontent of scale precipitated from brines containing dissolved silverand antimony, and that silver-containing scale, which is predominatelysilver antimonide, forms preferentially at locations of high turbulencewithin the liquid handling equipment from geothermal brines bearing themineral contents characteristic of brines from the geothermal fields ofBrawley, Calif. As a result of the preferential formation of silverscale in locations of high turbulence in the practice of this invention,a valuable silver-containing scale can be recovered by periodiallycollecting the scale deposited near the liquid handling equipmentproducing turbulence, such as pumps and valves.

Although silver recovery by the method of this invention can beaccomplished from any geothermal brine or other aqueous mediumcontaining significant concentrations of dissolved silver, the formationof silver or silver compounds is greatly increased when the brinecontains both silver and antimony, as is characteristic of the brinesfrom the Imperial Valley of California, especially those found in thevicinity of Brawley, particularly when the brine contains, for example,at least about 0.5 wppm of silver and at least about 1 wppm of antimony,for example, 0.5 to 1.5 wppm of silver and 1 to 2 wppm of antimony.

It is presently contemplated that dissolved bismuth and/or arsenic willalso increase the yield of silver in a manner analogous to thatdiscovered to be the case with antimony. It is further contemplated thateither arsenic, antimony or bismuth may be added to geothermal brines,particularly those deficient in such elements, so that an increasedrecovery of silver can be attained.

The silver-enriched scale recovered as a by-product from the processingof geothermal brines to control the deposition of scale may be processedby known means for recovery of elemental silver. Recovery of valuableminerals from geothermal brine has the particular advantage of enhancingthe overall economy of recovering heat energy from geothermal brine byprocesses which require a sudden reduction in temperature and pressureof highly saline, mineral laden brine.

While particular embodiments of the invention have been described, itwill be understood that the invention is not limited thereto since manyobvious modifications can be made. It is intended to include within thisinvention any such modifications as will fall within the scope of theappended claims.

We claim:
 1. A method for treating a pressurized geothermal brine at anelevated temperature containing silver and scale-forming constituents,said method comprising adding to said fluid sodium formate and awater-soluble acid or acid precursor and forming a silver-containingscale.
 2. A method as defined in claim 1 wherein hydrochloric acid isadded to said brine.
 3. A method as defined in claim 2 wherein saidbrine also contains dissolved antimony.
 4. A method for treating anaqueous geothermal fluid at an elevated temperature, said geothermalfluid containing dissolved silver and scale-forming salts, said methodcomprising adding sodium formate and hydrochloric acid to said fluid andreducing the pressure of said fluid at a sufficiently rapid rate to forma silver-containing scale.
 5. A method for treating an aqueousgeothermal fluid at an elevated temperature, said geothermal fluidcontaining dissolved silver and scale-forming salts, said methodcomprising adding hydrochloric acid and a reducing agent selected fromthe group consisting of sodium formate, sodium and ammonium thiosulates,elemental iron dust, elemental zinc dust and sodium dithionite.
 6. Amethod as defined in claim 5 wherein the acid is added in an amount soas to lower the pH of the fluid by between 0.1 and 0.5 pH unit.
 7. Amethod as claimed in claim 6 wherein the geothermal fluid containsdissolved antimony and at least some of the silver-containing scalecontains silver and antimony.
 8. A method as defined in claim 7 whereinsaid scale containing silver and antimony forms perferentially atlocations of high turbulence.
 9. A method for treating a pressurized,aqueous geothermal brine at an elevated temperature, said geothermalbrine containing dissolved silver and antimony and further containingscale-forming salts, said method comprising (1) adding to said fluidsodium formate and an amount of hydrochloric acid sufficient to lowerthe pH of the stream by between about 0.1 and 0.5 pH unit and (2)subjecting said brine to relatively rapid reduction in pressure so as toform a scale containing silver and antimony and further subjecting saidbrine to a zone of turbulence wherein a substantial proportion of saidscale forms.
 10. A method for treating a pressurized, aqueous geothermalbrine at an elevated temperature, said geothermal brine containingdissolved silver and antimony and further containing scale-formingsalts, said method comprising (1) adding to said fluid hydrochloric acidand a reducing agent selected from the group consisting of sodiumformate, sodium and ammonium thiosulfates, elemental zinc metal,elemental iron metal and sodium dithionite.